Like a dwarf galaxy, these galaxies are 99% dark matter (or an equivalent modified gravity effect). But, while a dwarf galaxy is 1% of the mass of the Milky Way, the new kind of galaxy, called "ultradiffuse" has a mass comparable to that of the Milky Way.
Their detection was made possible by a compound eye telescope using a special filter designed to prevent light scattering which makes ultradiffuse galaxies hard to see with conventional telescopes.
This kind of astronomy isn't cheap. Parts alone for the prototype "Dragonfly" satellite with 48 lens combined in a compound eye arrangement cost more than $576,000, in addition to countless hours of design and assembly. But, the price tag is still cheap by the standards of Big Science for a new kind of instrument that can see something never before seen with any other kind of telescope, let alone a ground based one.
A preprint of a paper describing one such discovery is as follows::
We report the discovery of three large (R29 >~ 1 arcminute) extremely low surface brightness (mu_(V,0) ~ 27.0) galaxies identified using our deep, wide-field imaging of the Virgo Cluster from the Burrell Schmidt telescope. Complementary data from the Next Generation Virgo Cluster Survey do not resolve red giant branch stars in these objects down to i=24, yielding a lower distance limit of 2.5 Mpc. At the Virgo distance, these objects have half-light radii 3-10 kpc and luminosities L_V=2-9x10^7 Lsun. These galaxies are comparable in size but lower in surface brightness than the large ultradiffuse LSB galaxies recently identified in the Coma cluster, and are located well within Virgo's virial radius; two are projected directly on the cluster core. One object appears to be a nucleated LSB in the process of being tidally stripped to form a new Virgo ultracompact dwarf galaxy. The others show no sign of tidal disruption, despite the fact that such objects should be most vulnerable to tidal destruction in the cluster environment. The relative proximity of Virgo makes these objects amenable to detailed studies of their structural properties and stellar populations. They thus provide an important new window onto the connection between cluster environment and galaxy evolution at the extremes.
Chris Mihos, Patrick R. Durrell, Laura Ferrarese, John J. Feldmeier, Patrick Côté, Eric W. Peng, Paul Harding, Chengze Liu, Stephen Gwyn, and Jean-Charles Cuillandre "Galaxies at the extremes: Ultra-diffuse galaxies in the Virgo Cluster" (July 24, 2015) (to appear in ApJ Letters).